Amblyopia (commonly known as lazy eye) is a condition of diminished visual form sense which is not as a result of any clinically demonstrable anomaly of the visual pathway and refers to a partial reversible loss of vision in one or both eye. There will be absence of any organic diseases of intra ocular structures i.e ocular media, retina, visual pathway etc.
It is the cause of temporary active cortical inhibition of the image of an object forms on the retina of suppressed eye, where another eye can fix and can form clear image on the retina. It can be defined as inadequate stimulation of the visual system during critical period of visual development in early childhood (<6 years) in which certain amblyogenic factors plays vital role in visual disturbance.
Amblyopia is one of the most important causes of vision impairment in the world, especially in children. Although its prevalence varies in different parts of the world, according to the article published in the pub in Jul 2018, it is mentioned that about 162-188 in 10,000 people suffer from amblyopia including children.
What causes amblyopia?
During the critical period of visual development, the visual pathway continuous to develop and our brain learns to interpret the signals which come from the eye. Following factors affect the visual signal and disturb the visual functions, which may be unilateral or bilateral and cause may be one or the combination of:
- Light deprivation:
There will be no visual stimulus to the retina because the light rays entering the eye is obstructed. It can be due to congenital cataract, traumatic cataract, complete ptosis, dense corneal opacity.
- Form deprivation( visual deprivation):
In this retina receives a defocused image as with refractive errors which occur in anisometropia. Anisometropic amblyopia occurs in the eye having a higher degree of refractive error than the fellow eye. It is more common in anisohypermetropic than the anisomyopic eye.
- Abnormal binocular interaction
Non-fusible images fall on each fovea as with strabismus (due to squinting eyes). Strabismus amblyopia results from prolong uniocular suppression with unilateral constant squint and fixating with normal eye.
Clinical characteristics (signs and symptoms):
- Visual acuity: It will be reduced, recognition acuity will be affected but the resolution acuity will not be affected. One can experience blurred vision and double vision.
- Crowding phenomenon: visual acuity will be less when tested with multiple letters charts than when tested with single optotype charts.
- Colour vision: usually normal but can affect in deep amblyopia with vision less than 6/36.
- An eye that wanders inward or outward.
- Eyes that appear to not work together.
- Poor depth perception:
May causes problems with,
- Hand-eye coordination
- Sports performance
- Maneuvering steps
- Driving skill
- Making crafts
- Squinting or shutting an eye.
- Head tilting.
- Loss of perception of movement
- Reduced contrast
Factors associated with an increased risk of lazy eye include:
- Premature birth
- Small size at birth
- Family history of lazy eye
- Developmental disability
How does amblyopia affect vision?
Normally,the image each eyes sends to the brain are identical ,fusible and focused. When the images sent to brain differs too much, the brain will learns to ignore the poor image sent by the amblyopic eye and sees with only the good eye.
The vision of the eye that the brain ignores becomes weaker from the lack of use and at a certain point of time, its becomes lazy and called a lazy eye.
Can we correct lazy eye?
Neuroscience has proven that the human brain can change at any age (neuroplasticity), so lazy eye is treatable at any age too, but the treatment should be done earlier as possible where the situation should not worsen. The chance of getting the vision back is too less. Treatment involves glasses, atropine drops eye patching, or vision therapy, which includes eye exercises, visual-motor processing activities, and neurological therapies.
- Glasses (refractive error correction):
Anisometropia, high astigmatism and bilateral high refractive error should be corrected.
- Eye patching:
Occlusion or eye patching, the most commonly used method of treating amblyopia. The normal eye, occluded and occlusion may be in the form of total light or form, or partial.
Aim of occlusion:
- Equalise visual acuity
- Achieve optimum visual acuity
- Central fixation
- Cycloplegic drugs
Where occlusion is not working or not tolerated, cycloplegic drugs may be used to blur the vision in the better eye, thereby giving the amblyopic eye more stimulus. Atropine 1% has been used once daily.
- Optical penalization: Blurring of vision of normal eye by using over plus lenses in spectacles.
- Pleoptics exercise: used in past to re-establish foveal fixation. It is a uniocular form of treatment using after image and Haidinger’s brushes to encourage foveal fixation with normal projection.
- Computerized vision therapy: using specially designed software has come into vogue for the treatment of amblyopia with controversial results. Prescribed as a supplementary treatment to occlusion therapy.
Does amblyopia get worsen?
Vision in the amblyopic eye may continue to decrease if left untreated. The image formed in amblyopic eye will be rejected by the brain because it will be non-fusible and defocused. So, the brain simply pays less and less attention to the image sent by the amblyopic eye. Eventually at one point the condition will be stable and the eye will be virtually unused and at this point it will be very difficult to treat the amblyopia effectively.
Is amblyopia preventable?
Yes, amblyopia is preventable if detected early and treated. Significantly unequal refractive error can be reduces and chances of one eye becoming amblyopia can be decrease and prevented.
Is the amblyopic eye blind?
No, amblyopic eye is not a blind eye. It sees the images but not clearly. Amblyopia mostly affects central vision but the peripheral vision is less affected.
But, if left untreated can cause permanent loss of vision
What are the consequences of amblyopia?
Permanent vision loss in the affected eye and also risk of permanent visual loss in the better eye, reported as 32.9 per 100,000 populations (Rahi et al. 2002). Presence of amblyopia interferes with schooling, works, lifestyle, sports and career choice because of poor vision, related impairment of motor tasks and binocular vision and defect of prehension including reaching and grasping the object.
Digital screen and Refractive error in Amblyopia
The digital screen is the displays flat panel that is present in mobiles, laptops, televisions, etc. which commonly use LCD screens that are based on light-emitting diode technology. Such a screen is the most common and leading cause of eye fatigue and strain in today’s scenario. It became our daily routine to use digital gadgets in everyday practice and works.
But most common bad practice is that parents allow their children to use the digital screen for long time without break.
Children will spend hours with their digital screen and don’t consider the strain and fatigue of their eyes and children will not complain or report headache, stress and fatigue to their parents.
This habit will increase for the long time and incidence of eye problems will increase with age.
However, so many studies have proven that constantly looking and fixing to the screen may cause eyes related problems in children.
The common problem is refractive error (myopia) but in children most common is anisohypermetropia and occult deviation i.e deviating eye called as squint.
Ocular deviation occurs in children who look at a fixed screen for a long period of time. On the other hand long hours near work and use of screen can trigger myopia in children and can accelerate it progression.
Also, some studies have shown that working too near to the digital screen may cause dryness, itching, and certain allergy, which may cause increased rubbing of the eyes in general. This continuous rubbing of the eye can lead or trigger the development and progression of astigmatism and myopia.
This rubbing of eye lead to the refractive error called myopia and astigmatism due to change in the corneal curvature of the eye.
If the refractive error progress in one or both the eyes brain will have difficulty to interpret the image seen by the eyes. Response from the brain to the respective eye will decrease as compare to the good eye.
Where both the eye can have high refractive error though, but the brain will see the image which will be clear and focus as compared to another eye. If this process will be for a long time brain will interpret or start to see from the good eye and another eye will be left behind without work.
If this problem, not treated for a long period of time. The brain will neglect the eye without work and lead to the suppression of the particular eye and will convert to the lazy eye called an amblyopic eye.
Can refractive error lead to amblyopia?
Yes, a refractive error can lead to an amblyopic eye, known as anisometropic amblyopia, which occurs in the eye having a higher degree of refractive error than the fellow eye. And it is more common in hypermetropic children than myopic children. 1-2 D hypermetropic anisometropia can/may cause amblyopia, where 3-4 D myopic anisometropia usually does not lead to amblyopia.
In isoametropic amblyopia, bilateral amblyopia occurs in children with bilateral uncorrected high refractive error, where both the eyes will have high refractive-error.